Technical Field
The present invention relates to a communication system in which dominant signals from a plurality of communication units connected to a communication line are likely to collide with each other.
Related Art
In a known communication system as disclosed in a patent document JP-B-2839054, for example, a plurality of communication units (hereinafter also referred to as nodes) are connected to a communication line. The plurality of nodes mutually transmits PWM (pulse width modulation) codes, and when transmission signals (dominant signals) collide with each other, the node transmitting a longer dominant pulse is ensured to be superior in arbitration.
In some communication methods, dominant signals outputted from a plurality of nodes are likely to collide with each other. Such communication methods may, for example, be the CSMA/CD (carrier sensor multiple access with collision detection) method as disclosed in the above patent document JP-B-2839054 or a master-slave type communication method in which a slave node performs data transmission in synchronization with the data transmission from a master node.
In the communication system mentioned above, dominant signals are transmitted from a plurality of nodes and may collide with each other. If a node stops transmission of a dominant signal under the condition that dominant signals collide with each other, the voltage outputted from this node to the transmission line may vary, causing drastic variation in the current flowing through the communication line.
For example, let us refer to FIG. 7 illustrating a general configuration of a communication system based on conventional art. This communication system includes a communication line 2, a master node 10 and a slave node 30. The master node 10 and the slave node 30 are connected to the communication line 2 configuring a communication bus. The master node 10 periodically transmits a dominant signal. In synchronization with the periodical transmission of a dominant signal from the master node 10, the slave node 30 switches between transmission and non-transmission of a dominant signal to thereby transmit serial data.
In this communication system, the nodes 10 and 30 include terminal resistors 12 and 32, transmission transistors 14 and 34, and communication circuits 20 and 40, respectively. Each of the terminal resistors 12 and 32 connects a power-supply line applied with a power-supply voltage Vc to the communication line 2. Each of the transmission transistors 14 and 34 connects the communication line 2 to a grounding line to transmit a dominant signal.
The terminal resistor 12 of the master node 10 has a resistance which is set to a value (e.g., 1 kΩ) much smaller than a resistance (e.g., 30 kΩ) of the terminal resistor 32 of the slave node 30.
FIG. 8 is a timing diagram illustrating the operation of this communication system. As shown in FIG. 8, the communication circuit 20 of the master node 10 periodically outputs a drive voltage S1 (high level) to the transistor 14 via a resistor 18. Thus, the transistor 14 is periodically turned on and permitted to transmit a dominant signal (ground potential).
On the other hand, the communication circuit 40 of the slave node 30 monitors voltage variation of the communication line 2 via a comparator 36 to determine whether or not a dominant signal has been outputted from the master node 10.
Upon detection of the output of a dominant signal from the master node 10 (time t1 and only when transmission data has a value 1, the communication circuit 40 turns on the transistor 34 for a predetermined period (from time t2 to time t4) to output a dominant signal (ground potential). In this case, the communication circuit 40 prolongs the low-level period of the communication line 2 more than the low-level period when the transmission data has a value 0.
The communication circuit 20 of the master node 10 monitors voltage variation (High-Low) of the communication line 2 via a comparator 16. Thus, the communication circuit 20 of the master node 10 is able to detect the data transmitted from the slave node 10 on the basis of the duty ratio of High-Low of the communication line 2.
In this way, in the communication system shown in FIG. 7, the slave node 30 synchronizes with a dominant signal from the master node 10 to transmit a dominant signal when the transmission data has a value 1. However, this way of transmission causes collision between dominant signals outputted from the nodes 10 and 30.
When such a collision occurs (time t2), the transmission transistors 14 and 34 are both in a turned-on state. Therefore, no potential difference is caused between the nodes 10 and 30 in the communication line 2 and thus the current flowing through the communication line 2 will become zero.
After that, however, the communication circuit 20 of the master node 10 switches the drive voltage S1 of the transistor 14 to a low level and stops the output of a dominant signal (time t3). Then, current flows from the terminal resistor 12 of the master node 10 to the transistor 34 of the slave node 30 via the communication line 2.
In this case, in terms of the path of current flowing through the terminal resistor 12, the transistor 14 is merely replaced by the transistor 34. Accordingly, the slave node 30 is in a state where the communication line 2 is connected to the grounding line via the transistor 34. Therefore, the slave node 30 is not able to control the current flowing through the communication line 2.
As a result, at time t3 when the transistor 14 of the master node 10 is turned off as mentioned above, the current flowing through the communication line 2 steeply varies and this steep current variation causes large radiation noise.
The radiation noise can be reduced by suppressing the variation per unit time of current (gradient of current shown in FIG. 8) flowing through the communication line 2. However, in the slave node 30, the potential of the communication line 2 is fixed to a potential of a level of outputting a dominant signal (the ground potential here). Therefore, the drive voltage of the transistor 34 cannot be controlled to suppress the current variation, as is done in the switching between output and stop of the dominant signal.